SEGMENTED REFRACTION OF THE CRYSTALLINE LENS AS A PREREQUISITE FOR THE OCCURRENCE OF MONOCULAR POLYPLOPIA, INCREASED DEPTH OF FOCUS, AND CONTRAST SENSITIVITY FUNCTION NOTCHES

Theoretical computations of modulation transfer functions (MTF's) of t he optical system of the human eye have shown that irregular aberratio n consisting of a small circular segment with refractive power slightl y different from the surround introduces at higher spatial frequencies (> 20 cpd) an enhancement of the retinal image contrast on flanks of the optimum-focus plane. When the pupil size is larger than 3 mm, enha ncement is substantial; as a result, multiple foci appear at the affec ted, higher spatial frequencies and generate a greater depth of focus. The contrast enhancement also produces troughs on either flank of the optimum-focus plane. With slight coincident defocus (+/-0.5 diopter) of the retinal image of a sine-wave grating, notches in the MTF curves , with a contrast reduction in the intermediate frequency range of a f actor of 2 to 3 and a low cutoff spatial frequency of similar to 3 cyc les/deg, are produced. In our theoretical study, multiple foci, monocu lar polyplopia, and increased depth of focus are implicated in the gen eration of contrast sensitivity function (CSF) notches. It is demonstr ated that CSF notches of optical origin can extend to lower spatial fr equencies (< 10 cycles/deg). As a result, before the presence of a CSF notch can be attributed to neurological abnormality, optical factors, including irregular aberrations, must be eliminated.